- Memory Mapping
- Signals
- Semaphores
- Message Queues
- Shared Memory
- Named Pipes
- Pipes
- System V & Posix
- Related System Calls (System V)
- Overlaying Process Image
- Other Processes
- Process Resources
- Process Groups, Sessions & Job Control
- Child Process Monitoring
- Process Creation & Termination
- Process Image
- Process Information
- Overview
- Home
Useful Resources
Selected Reading
- Who is Who
- Computer Glossary
- HR Interview Questions
- Effective Resume Writing
- Questions and Answers
- UPSC IAS Exams Notes
Process Groups, Sessions & Job Control
In this chapter, we will get famipar with Process Groups, Sessions and Job Control.
Process Group − Process group is a collection of one or more processes. A process group constitutes of one or more processes sharing the same process group identifier (PGID). A process group ID (PGID) is of the same type (pid_t) as the process ID. A process group has a process group leader, which is the process that creates the group and whose process ID becomes the process group ID of the group.
Sessions − It is a collection of various process groups.
Job Control − This permits a shell user to simultaneously execute multiple commands (or jobs), one in the foreground and all remaining in the background. It is also possible to move the jobs from the foreground to the background and vice-versa.
Let us understand this with the help of example program/s using shell (BASH).
Shell script (in BASH) to perform basic commands (date, echo, sleep and cal) named basic_commands.sh
Shell script (in BASH) to perform basic commands (ps, echo)
#!/bin/bash #basic_commands.sh date echo "Now sleeping for 250 seconds, so that testing job control functionapty is smooth" sleep 250 cal
#!/bin/bash #process_status.sh ps echo "Now sleeping for 200 seconds, so that testing job control functionapty is smooth" sleep 200 ps
Use chmod command to give the file the execute permissions. By default, the normal file would get only read and write permissions and not execute permissions.
To stop the current running process, you need to enter CTRL+Z. This gives you a job number. The job can be resumed either in the foreground or the background. If needed, to resume the job in the foreground use ‘fg’ command. If needed, to resume the job in the background, use ‘bg’ command. By using this, it would run only the last stopped process. What if you want to start other than the last stopped process? Just use the job number after fg or bg (say bg %2 or bg %3, etc). If the running job is in the background, you can run any other tasks in the foreground. To get the pst of jobs, use command, jobs. It is also possible to terminate the process either with CTRL+C or kill command. You can pass the job number while using the kill command.
Check the following output which demonstrates stopping the jobs, moving the jobs from the foreground to the background and vice versa, terminating the jobs, etc.
chmod u+x basic_commands.sh chmod u+x process_status.sh ./basic_commands.sh Wed Jul 5 18:30:27 IST 2017 Now sleeping for 250 seconds, so that testing job control functionapty is smooth ^Z [1]+ Stopped ./basic_commands.sh ./process_status.sh PID TTY TIME CMD 2295 pts/1 00:00:00 bash 4222 pts/1 00:00:00 basic_commands. 4224 pts/1 00:00:00 sleep 4225 pts/1 00:00:00 process_status. 4226 pts/1 00:00:00 ps Now sleeping for 200 seconds, so that testing job control functionapty is smooth ^Z [2]+ Stopped ./process_status.sh jobs [1]- Stopped ./basic_commands.sh [2]+ Stopped ./process_status.sh fg ./process_status.sh ^Z [2]+ Stopped ./process_status.sh fg %2 ./process_status.sh ^Z [2]+ Stopped ./process_status.sh fg %1 ./basic_commands.sh ^Z [1]+ Stopped ./basic_commands.sh jobs [1]+ Stopped ./basic_commands.sh [2]- Stopped ./process_status.sh bg %2 [2]- ./process_status.sh & fg ./basic_commands.sh ^Z [1]+ Stopped ./basic_commands.sh jobs [1]+ Stopped ./basic_commands.sh [2]- Running ./process_status.sh & fg %2 ./process_status.sh ^Z [2]+ Stopped ./process_status.sh jobs [1]- Stopped ./basic_commands.sh [2]+ Stopped ./process_status.sh kill %1 %2 [1]- Stopped ./basic_commands.sh [2]+ Stopped ./process_status.sh [1]- Terminated ./basic_commands.sh [2]+ Terminated ./process_status.shAdvertisements